1. Field of the Invention
The present invention generally relates to the technology of electron beam lithography, and more specifically to a method for proximity effect correction in high voltage electron beam lithography systems. The invention may also advantageously be applied to proximity correction of very dense patterns at lower beam voltages.
2. Description of the Prior Art
U.S. Pat. No. 4,264,711 issued Apr. 28, 1981, to Greeneich entitled METHOD OF COMPENSATING FOR PROXIMITY EFFECTS IN ELECTRON-BEAM LITHOGRAPHY describes a process of constructing microelectronic circuits on a semiconductor chip wherein electron-beam lithography is utilized to fabricate high resolution resist patterns. The resolution however, is limited by proximity effects which are due to scattering of the electron-beam as it passes through the resist. In the disclosed method, those proximity effects are compensated for by making the energy that is absorbed by the resist from the electrons, substantially greater at the perimeter of each shape in the pattern than at the interior of those shapes by using extra energy dosage at the perimeter.
U.S. Pat. Nos. 4,426,564 and 4,504,558 issued Jan. 17, 1984, and Mar. 12, 1985, to Bohlen et al. entitled METHOD OF COMPENSATING THE PROXIMITY EFFECT IN ELECTRON BEAM PROJECTION SYSTEMS describes a system for compensating scattering losses of electrons in photoresists (proximity effect) which influence electron beam lithography by altering the pattern geometry and exposing selected partial areas of a pattern to an additional irradiation dosage in a second exposure step. For that purpose, a specific mask with corresponding correction openings can be used which is applied with the same, or with a different electron beam intensity. In a particularly advantageous manner, the correction of the proximity effect can be achieved when complementary masks are used; the correction openings for the partial areas of the once complementary mask are arranged in the other complementary mask. The proximity effect is then corrected without an additional exposure step. For measuring the proximity effect, a photo-optical process is suggested where line patterns with decreasing ridge width in the photoresist are defined through electron beam projection, and where the developing process of the photoresist is discontinued prematurely. The ridge edges which in the presence of proximity effect are asymmetrical, can be easily detected under the microscope.
In U.S. Pat. No. 4,498,010 issued Feb. 5, 1985, to Biechler et al., entitled VIRTUAL ADDRESSING FOR E-BEAM LITHOGRAPHY, a technique is performed in a fixed address particle beam lithographic system where the writing is performed in the normal manner for writing a pattern, for example, a stripe on a resist having a selected feature width except that an additional row of alternate pixels is written either before or after the selected feature is written. The alternate pixels, when the resist is developed, will provide a feature width of approximately 1/2 a pixel wider than the selected feature width due to blurring of the latent image caused by scattering of the particle beam within the resist. Thus, the resolution of selectable feature widths is enhanced with little or no loss of throughput. The same technique can also be utilized to lengthen a feature by 1/2 a pixel width. The technique is disclosed primarily in a raster scan machine but also disclosed is the technique in a vector scan machine. Also disclosed is a flow chart showing the invention used while preparing the data to be written by the machine.
In U.S. Pat. No. 4,520,269, issued May 28, 1985, to Jones entitled ELECTRON BEAM LITHOGRAPHY PROXIMITY CORRECTION METHOD, proximity effect is reduced or eliminated by breaking each shape of a lithographic exposure pattern into two parts, a perimeter part having a width on the order of the lithographic exposure pattern minimum line width and the remaining central part or parts (if any) which are completely surrounded by the perimeter part. The lithographic exposure pattern is then modified by moving or setting back the edges of each central part away from the perimeter part which surrounds it (similar to reducing the size of the central part) to form a nominally unexposed band separating each central part from the perimeter part which surrounds it.
The width of the nominally unexposed band in the modified exposure pattern is preferably chosen as large as possible so long as the condition is met that upon developing a radiation sensitive layer directly exposed to the modified exposure pattern, the nominally unexposed band develops (i.e., dissolves, resists dissolution, or is otherwise modified) substantially as it were also exposed. The nominally unexposed band is exposed, in fact, by electrons scattered from the directly exposed part(s) of the shape (the perimeter part plus the central part, if any). The width of the nominally unexposed band is preferably about twice the edge bias applied to outside edges of each shape.
European Patent Application 83106013.2, Publication Number 0097903, by Komatsu, filed June 20, 1983, entitled METHOD OF ELECTRON BEAM EXPOSURE, describes a method of electron beam exposure comprising selectively exposing a resist film, on a substrate a plurality of times with an electron beam whose dose is lower than a desired dose sufficient to produce a difference in molecular weight between the exposed area and nonexposed area, the cumulative dose corresponding to said desired dose.
European Patent Application 83303812.8, Publication Number 0098177, filed June 30, 1983, by Osada, entitled SCANNING ELECTRON-BEAM EXPOSURE SYSTEM, describes a scanning electron-beam exposure system for exposing a desired rectangular area which is larger than a predetermined maximum rectangle wherein the desired rectangular area is divided into a plurality of areas. Two sets of such divided areas are shifted two-dimensionally from each other. The one set of divided areas are exposed with half of a predetermined electron-beam dose and the other set of divided areas are also exposed with half of the predetermined dose. This serves to reduce undesirable effects at boundaries between scanned areas.
European Patent Application 83302399.7, Publication Number 0097417, filed Apr. 28, 1983, by Owen, entitled ELECTRON BEAM LITHOGRAPHY, describes an electron beam lithography system wherein a beam of incident electrons exposes a preselected circuit pattern in a thin resist layer deposited on top of a substrate to be etched. Some of the electrons scatter from the substrate back into the resist layer producing an undesired exposure which produces variable resolution of features. In accordance with the disclosed technique, the region of the resist which is complementary to the desired circuit pattern is also exposed by an electron beam which has been adjusted to produce an exposure approximating that due to backscattering. This additional exposure removes the spatial variability in resolution attainable by the electron beam lithography.
European Patent Application 85303998.0, Publication Number 0166549, filed June 5, 1985, by Pavkovich, entitled METHOD FOR PROXIMITY EFFECT CORRECTION IN ELECTRON BEAM LITHOGRAPHY SYSTEMS, describes a method for electron beam lithography writing of a micro-miniature pattern P(x,y) in a resist-coated workpiece, the method comprising the steps of calculating, at a plurality of grid points on the workpiece, the electron dosage required to produce uniform exposure of the pattern P in accordance with the expression ##EQU1## where S represents the backscattered exposure distribution produced by an incident electron beam, and A, B and .beta. are constants. The pattern is then written on the workpiece with the electron beam while varying the applied electron dosage in accordance with the calculated values of required electron dosage, in a preferred embodiment, the electron dosage applied by the electron beam lithography system is varied between a number of discrete levels. The features of the pattern P are partitioned into subfeatures in accordance with the calculated values of required electron dosage. The subfeatures are then assigned to one of the discrete dosage levels of the system.
Russian Patent Application 938339, publication date June 23, 1982, by Ulianov, entitled METHOD OF ELECTRONOLITHOGRAPHY, describes a technique to eliminate the influence of scattered electrons during the exposure of a resist in electron beam lithography by surrounding the border areas between the semiconductor substrate and resist with an electrical field which deflects electrons toward the semiconductor substrate.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 9, February 1978, at page 3809, the publication "Partitioning E-Beam Data For Proximity Corrections" by T. P. Chang et al. describes an algorithm for solving the proximity effect problem due to the partial exposure of resist near a shape written by an E-beam due to electron scattering in the resist and substrate. The shape written near to the original one should have its exposure corrected because of the fact that some of the resist in which it is written has been partially exposed in writing the first shape. There are two steps to this method for solving this problem:
1. The second shape must be subdivided into two (or more) shapes so that it now consists of a shape which lies on unexposed resist and one which lies on the resist partially exposed by writing the original shape which preceded it. PA1 2. The subshapes can now be given different exposures, depending on whether they lie in partially exposed or unexposed resist. PA1 1. The outside edges of the shapes are separated from the interior, reflecting the observation that dose assignment to the edges is far more critical than dose assignment to the interior. PA1 2. The interior shapes are assigned an empirically determined dose and are not corrected any further, dramatically reducing proximity correction algorithm processing time. PA1 3. The edges are divided into coarse segments of 40-50 micrometers to minimize the number of neighboring shapes and reduce the overall execution time. PA1 4. The neighborhood of shapes in the immediate vicinity of each coarse segment is built by including all shapes within a predefined radius around the coarse segment. This occurs in an algorithmically reproducible manner, thus assuring that similar shapes in similar environments will be assigned similar doses. PA1 5. All of the coarse segments are subdivided further to provide more accurate dose assignment in small local areas. PA1 6. The proximity correction algorithm assigns a dose to each subdivided coarse segment. PA1 7. After dose assignment is completed, shapes with the same dose are merged together and an electron writing beam pattern is produced.
In IBM Technical Disclosure Bulletin, Vol. 26, No. 12, May 1984, at page 6291, the publication by W. J. Guillaume et al. entitled "Proximity Correction Technique for Electron-Beam Tools" points out that proximity correction algorithms are required to vary electron-beam dose levels to compensate for intra proximity effects and tool focus deviations and discloses a method for determining which shapes are to be presented to the proximity correction algorithm for dose assignment. The technique provides smoother dose transition, consistent dose assignment, and dramatically reduces proximity correction algorithm processing time.
The method operates on electron-beam mask writing shape data with the following steps.
In the publication entitled "Proximity effect in electron-beam lithography", by T. H. P. Chang in J. Vac. Sci. Technol., Vol. 12, No. 6, November/December 1975, at page 1271, a simple technique for the computation of the proximity effect in electron-beam lithography is presented. The calculations give results of the exposure intensity received at any given point in a pattern area using a reciprocity principle. Good agreement between the computed results and experimental data was achieved.
The publications in J. Appl. Phys., 50 (6), June 1979, at pages 4371, 4378 and 4383, "Corrections to proximity effects in electron beam lithography. (I. Theory) (II. Implementation) (III. Experiments)" by M. Parikh disclose three correction techniques for proximity effects. The self-consistent technique computes the incident electron dose such that identical average specific exposure occurs in each written shape of the pattern. A unique solution, that depends only on the form and on the magnitude of proximity function, is obtained. The unaddressed-region compensation technique attempts to compensate for proximity effects in regions between shapes. This, however, leads to computational complexities and impracticalities. The shape-dimension adjustment technique attempts to compute dimension of exposed shapes such that the shapes developed in the resist will have the designed dimension. A set of nonlinear (and impractical) equations are obtained in this case. The implementation of these techniques and the experimental results obtained therefrom are the subject of the two succeeding papers (II and III).
In the publication, "Proximity effect correction for electron beam lithography by equalization of background dose" by G. Owen et al., J. Appl. Phys., 54 (6) June 1983, at page 3573, it is disclosed that compensation for the proximity effect in electron lithography can be achieved by equalization of the backscattered dose received by all pattern points. This is accomplished by exposing the reverse tone of the required pattern with a calculated beam diameter and electron dose.
The publication "Proximity effect correction calculations by the integral equation approximate solution method" by J. M. Pavkovich, J. Vac. Sci. Technol., B4 (1) January/February 1986, at page 159 describes a method which provides a relatively accurate approximate solution to the integral equation which is easy to calculate and which provides information on where features should be fractured to obtain good dose compensation.
The publication "Measurements of electron range and scattering in high voltage e-beam lithography" by P. M. Mankiewich et al., J. Vac. Sci. Technol., B3 (1) January/February 1985, at page 174, states that proximity effect from electron scattering is a major limitation to using e-beam lithography for high density, submicron patterns At higher voltages, the proximity effect becomes a relatively uniform background dose without the local pattern distortions characteristic of conventional e-beam lithography at lower voltages. However, the total magnitude of the effect is not reduced and its variation over longer distances remains a serious problem.
The publication, "Proximity correction on the AEBLE-150" by O. Otto et al., J. Vac. Sci. Technol., B6 (1) January/February 1988, at page 443, describes a variant of Parikh's self-consistent method. Rather than requiring equal average exposure for every shape, this method requires equal exposure at a single "test point" in each shape or sub-shape. The locations of the test points are chosen according to specific rules. As in Parikh's method, a set of simultaneous equations is constructed and solved numerically to obtain the doses to be applied to each shape.
The publication, "Proximity correction on a vector scan e-beam machine by dosage variation" by B. J. Hughes et al., Microelectronic Engineering, 9 (1-4) May 1989, at page 243, describes yet another variation on Parikh's self-consistent method. As in Parikh's method, the doses are chosen to equalize the total average exposure received by each shape or sub-shape. The influence coefficients which define the resulting simultaneous equations are computed by a technique of "dose sampling" at meshes of grid points which are placed over each shape.
The publication, "Proximity effect correction for high-voltage electron beam lithography" by T. Abe et al., J. Appl. Phys., 65 (11) June 1989, at page 4428, describes an approximate dose correction method which is especially effective for a high acceleration voltage. An approximate formula is described which expresses the backscatter dose correction at any point as a simple sum of contributions from nearby shapes.